Method and apparatus for detecting combustion instability in continuous combustion systems

ABSTRACT

An apparatus and method to sense the onset of combustion stability is presented. An electrode is positioned in a turbine combustion chamber such that the electrode is exposed to gases in the combustion chamber. The electrode may be integrated with an igniter. A control module applies a voltage potential to the electrode and detects a combustion ionization signal and determines if there is an oscillation in the combustion ionization signal indicative of the occurrence of combustion stability or the onset of combustion instability. The control module broadcasts a notice if the parameters indicate the combustion process is at the onset of combustion instability or broadcasts an alarm signal if the parameters indicate the combustion process is instable. Combustion parameters are adjusted to drive the combustion process towards stability.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation of U.S. Pat. No. 6,993,960,Ser. No. 10/411,167 filed Apr. 10, 2003, which is a continuation-in-partof U.S. Pat. No. 7,096,722, Ser. No. 10/329,664, filed Dec. 26, 2002.

FIELD OF THE INVENTION

The present invention relates generally to continuous combustionsystems, and more particularly relates to such systems operating nearthe onset of combustion instability.

BACKGROUND OF THE INVENTION

Continuous combustion systems such as gas turbine engines are used in avariety of industries. These industries include transportation, electricpower generation, and process industries. During operation, thecontinuous combustion system produces energy by combusting fuels such aspropane, natural gas, diesel, kerosene, or jet fuel. One of thebyproducts of the combustion process is emission of pollutants into theatmosphere. The levels of pollutant emissions are regulated bygovernment agencies. Despite significant reductions in the quantity ofenvironmentally harmful gases emitted into the atmosphere, emissionlevels of gases such as NO_(x), CO, CO₂ and hydrocarbon (HC) areregulated by the government to increasingly lower levels and in an everincreasing number of industries.

Industry developed various methods to reduce emission levels. One methodfor gaseous fueled turbines is lean premix combustion. In lean premixcombustion, the ratio between fuel and air is kept low (lean) and thefuel is premixed with air before the combustion process. The temperatureis then kept low enough to limit the formation of nitrous oxides (whichoccurs primarily at temperatures above 1850 K). The premixing alsodecreases the possibility of localized fuel rich areas where carbonmonoxides and unburnt hydrocarbons are not fully oxidized.

One of the more difficult challenges facing manufacturers of lean premixgas turbines and other continuous combustion systems is the phenomenonof combustion instability. Combustion instability is the result ofunsteady heat release of the burning fuel and can produce destructivepressure oscillations or acoustic oscillations. In lean premix gasturbines, combustion instability can occur when the air-fuel ratio isnear the lean flammability limit, which is where turbine emissions areminimized. In general, the air/fuel ratio of the premixed fuel flowshould be as lean as possible to minimize combustion temperatures andreduce emissions. However, if the air/fuel ratio is too lean, the flamewill become unstable and create pressure fluctuations. The typicalmanifestation of combustion instability is the fluctuation of combustionpressure sometimes occurring as low as+/−1 psi at frequencies rangingfrom a few hertz to a few hundred hertz. Depending on the magnitude andfrequency, this oscillation can create an audible noise which issometimes objectionable, but a much more serious effect can becatastrophic failure of turbine components due to high cycle fatigue.The most severe oscillations are those that excite the naturalfrequencies of the mechanical components in the combustion region, whichgreatly increases the magnitude of the mechanical stress.

Most continuous combustion systems are commissioned in the field withsufficient safety margin to avoid entering an operating regime wherecombustion instabilities can occur. However, as components wear out orfuel composition changes, the combustion process can still becomeunstable.

BRIEF SUMMARY OF THE INVENTION

The invention provides an apparatus and method to sense the presence ofcombustion instability, even at very low levels, so that an operator ora closed loop control system can take mitigating action to eitherrestore combustion stability or shut down the combustion process (in theturbine or afterburner).

An ion sensor such as an electrode is positioned in the combustionchamber of a turbine combustion system at a location such that thesensor is exposed to gases in the combustion chamber, and in particularthose gases containing free ions that are produced during combustion. Avoltage is applied to the sensor to create an electric field from thesensor to a designated ground (e.g., a chamber wall) of the combustionchamber. The voltage is applied in one embodiment such that the electricfield radiates from the sensor to the designated ground of thecombustion chamber. If free ions are present in this field, a small ioncurrent will flow. The magnitude of the ion current gives an indicationof the density of ions. A control module detects and receives from thesensor a combustion ionization signal and determines if there is anoscillation in the combustion ionization signal indicative of theoccurrence of combustion instability or the onset of combustioninstability. This provides the ability to indirectly monitor pressureoscillations by inferring a pressure oscillation without requiring anexpensive and unreliable pressure transducer to be installed in thefield. The oscillation magnitude of the ion signal is correlated to apressure oscillation magnitude and stored in the controller memory.

The control module applies a voltage to the ion sensor during thecombustion process, measures the ion current flowing between the sensorand the designated ground of the combustion chamber, and compares theionization current oscillation magnitude and oscillation frequencyagainst predetermined parameters and broadcasts a signal if theoscillation magnitude and oscillation frequency are within a combustioninstability range. The parameters include an oscillation frequency rangeand an oscillation magnitude.

The signal is broadcast to indicate combustion instability if theoscillation frequency is within a critical range for a given combustionsystem (e.g., the range of approximately 250 Hz to approximately 300 Hzfor a critical frequency of 275 Hz) and/or the oscillation magnitude ofpressure can be inferred from the ion signal to be above a firstthreshold relative to a steady state magnitude (e.g., ±2 psi). Thesignal is broadcast to indicate the onset of combustion instability ifthe oscillation frequency is within the critical range and/or theoscillation magnitude is above a second threshold relative to a steadystate magnitude. In response to receiving the signal, a mitigatingaction is taken such as enriching the air/fuel ratio, adjusting the flownozzle geometry, or other type of mitigating action.

These and other advantages of the invention, as well as additionalinventive features, will be apparent from the description of theinvention provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention, andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a diagram illustrating the components of the present inventionin a portion of a turbine system where the ion sensor electrode isintegral with an igniter;

FIG. 2 is a diagram illustrating an ion sensor electrode in a retractionmechanism;

FIG. 3 is a diagram illustrating an ion sensor electrode in a fuelnozzle;

FIG. 4 is a diagram illustrating an igniter used solely as an ionsensor;

FIG. 5 is a diagram illustrating an ion sensor of the present inventionin a system having combustion instability;

FIG. 6 is a diagram illustrating an ion sensor in accordance with thepresent invention in a system having combustion instability in acombustion chamber having electrically insulated walls;

FIG. 7 is a graphical illustration of the output of a pressure sensorand ion current illustrating that ion current oscillations correspond topressure oscillations in a combustion chamber;

FIG. 8 is a diagram illustrating that the dominant frequencies of ioncurrent oscillations track surges in pressure oscillations in acombustion chamber;

FIG. 9 is a flow chart illustrating the steps the present inventionperforms in tracking the onset of combustion instability or combustioninstability in accordance with the present invention; and

FIG. 10 is a diagram illustrating the components of FIG. 1 in a turbineenvironment operating in a closed loop mode to detect combustioninstability.

While the invention will be described in connection with certainpreferred embodiments, there is no intent to limit it to thoseembodiments. On the contrary, the intent is to cover all alternatives,modifications and equivalents as included within the spirit and scope ofthe invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method and apparatus to sensecombustion instability and/or the onset of combustion instability in acombustion region of a continuous combustion system such as a gasturbine, industrial burner, industrial boiler, or afterburner utilizingionization signals. The invention may be used with any hydrocarbonfuels, such as liquid or gaseous fuels, that produce free ions in theflame when the fuel is burned. The magnitude of the free ions in theflame is proportional to the concentration of hydrocarbons, andtherefore the measured ion current is also proportional to the magnitudeof free ions. Oscillations in the flame produce oscillations in thehydrocarbons, which in turn, results in oscillations in the ionizationsignal. When those ion current oscillations have been properlycorrelated to pressure oscillations, the ion signal gives a very clearindication of the pressure oscillations. The oscillation of the ionsignal is typically correlated in a laboratory environment with pressureoscillations as measured by a pressure transducer to properly interpretthe magnitude of the ion signal oscillation to a corresponding pressureoscillation. The invention detects the frequency and magnitude ofoscillations in the ionization signal and provides an indication whenthe frequency and magnitude of the ionization signal oscillation areabove selected thresholds.

Turning to the drawings, wherein like reference numerals refer to likeelements, the invention is illustrated as being implemented in asuitable turbine environment. FIG. 1 illustrates an example of asuitable turbine environment 100 on which the invention may beimplemented. The turbine environment 100 is only one example of asuitable turbine environment and is not intended to suggest anylimitation as to the scope of use or functionality of the invention. Forexample, the invention may be implemented in an afterburner, industrialburner, industrial boiler, and the like. Neither should the turbineenvironment 100 be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary operating environment 100.

With reference to FIG. 1, an exemplary system for implementing theinvention includes electronic module 102, fuel nozzle 104, andcombustion chamber 106. The fuel nozzle 104 is mounted to the combustionchamber 106 using conventional means. The fuel nozzle 104 is typicallymade of conducting material and has an inlet section 108, an outlet port110 that leads into combustion chamber 106 and a center body 112. Anigniter is used to ignite the fuel mixture in the combustion regionafter the air and fuel are mixed in a pre-mix swirler 114. Inafterburners, the air enters combustion chamber 106 through separatepassages and a fuel nozzle passage is used to introduce fuel in thecombustion chamber 106. The operation of the turbine is well known andneed not be discussed herein.

The electronic module 102 may be a separate module, part of an ignitioncontrol module or part of an engine control module. The electronicmodule 102 includes a power supply 130 for providing a controlled ac ordc voltage signal to the electrode 302 when commanded by processor 132.Processor 132 commands the power supply to provide power to theelectrode 302, receives ion current signals from electrode 302 viaconditioning module 136, performs computational tasks required toanalyze the ion signals to determine the onset of combustion instabilityand combustion instability, and communicates with other modules such asan engine control module through interface 134. Conditioning module 136receives signals from the electrode 302 via lines 138 and performs anyrequired filtering or amplification. The electrode 302 may be part of anigniter 300 or may be a separate ion sensor unit.

It should be noted that other types of ion current sensors may be usedin accordance with the present invention. For example, the electrodesdescribed in U.S. Pat. No. 6,429,020 and U.S. patent application Ser.No. 09/955,582 filed on Sep. 18, 2001, hereby incorporated by referencein their entireties, may be used. Additionally, the igniter of a gasturbine or industrial burner can be used to sense ion current in asimilar manner as with spark plugs on reciprocating gas engines. Theigniter is used for ignition only at startup and therefore is availablethe rest of the time for ion sensing and provides the advantage in thatthe igniter is already installed in the combustion chamber. An ignitercan be mounted in a combustion chamber of a gas turbine or industrialburner in a variety of ways. For example, the igniter can be placed at afixed location near the fuel nozzle (see FIG. 1), at a moving locationwith a retraction mechanism (see FIG. 2), integrated into the fuelnozzle (see FIG. 3), in a fixed location chosen not for ignitionpurposes but for ion sensing (see FIG. 4), and in a closed chamber forignition purposes only with a separate fuel supply (not shown).

Turning back to FIG. 1, placement of the igniter 300 in the cooling airstream of the system allows the igniter to tolerate the hightemperatures of the combustion chamber. It is also possible for theigniter 300 to be placed such that it is close enough to the flame 140to sense the ion field 150 but far enough removed to avoid failure fromhigh temperature. The ion current flows between the center electrode 302of the igniter and the grounded case 304 of the igniter. The igniter 300is connected to electronic module 102.

Turning now to FIG. 2, an alternate mounting arrangement for the igniter300 is on a sliding retraction mechanism 400 that positions the igniter300 in an ideal location for ignition but then retracts it whencombustion pressure and temperature rise. The retraction mechanism 400is common for large industrial turbines in that it keeps the igniter 300safely away from the flame, and therefore out of the ion field 150. Theelectronic module (or a separate module) of the present inventioncontrols the position of the retraction mechanism 400 to keep the tip306 of the igniter 300 in the ion field 150 and away from the flame 140,maintaining a safe operating temperature of the igniter 300. A morecomplex arrangement may be necessary which actively positions theigniter 300 through a control loop such that it is always in ameasurable ion field while monitoring the igniter temperature via athermocouple or other temperature measuring device near the tip. Thecontrol loop may need to cycle the igniter 300 in and out to take ionmeasurements momentarily and then retract for a period of time to coolthe igniter if the temperature of the igniter is above a thresholdtemperature or rapidly approaches a threshold temperature. Thistemperature control and actuator driver function may be added toelectronic module 102 as illustrated in FIG. 2 as a solenoid controlledby coil 200 or as a stand-alone module.

Turning now to FIG. 3, a further alternate mounting arrangement for theigniter is in the fuel nozzle at the discharge end next to thecombustion chamber 106. The igniter 300 is placed in the fuel nozzle 104such that cooling air or fuel flows around it and maintains a safeoperating temperature. The igniter tip 306 is exposed to the combustionchamber and is capable of reaching an ignitable mixture with theignition plasma as described in U.S. Pat. No. 4,938,019 to Angell et.al. An igniter in this position is capable of serving as an ion sensorafter ignition has occurred. FIG. 4 illustrates a further embodimentwhere the igniter 300 is located in the combustion chamber 106 away fromthe fuel nozzle 104 at a location where ions may be present, but insmaller concentrations depending on flame conditions and location. Inthis embodiment, the igniter 300 is used only as an ion sensor to detectthat the flame has moved away from the fuel nozzle by sensing anincrease in the ion current flowing between the center electrode 302 andthe grounded case 304. Additionally, other types of electrodes may beused that are capable of sensing ion current in continuous combustionsystems. In the description that follows, the electrode 302 shall beused for the purpose of describing the operation of the invention. It isrecognized that the electrodes described herein and other types ofelectrodes may be used with the invention.

Turning now to FIG. 5, during normal combustion, the flame 140 producesfree ions and the electrode 302 will have an ion current flow when avoltage is applied to the electrode 302. Ion current will flow betweenthe electrode 302 and ground (e.g., the chamber wall). The magnitude ofthe ion current flow will be in proportion to the concentration of freeions in the combustion process. When a voltage potential is applied toelectrode 302, an electric field potential 142 is established betweenthe electrode 302 and the remaining components in the combustionchamber. For combustion chambers having walls that are electricallyinsulated or are poorly grounded, a grounding strip is used to provide areturn path to enhance the flow of ion current. For example, FIG. 6shows a grounding strip 320 providing the return path for the ioncurrent 144 to flow between the electrode 302 and ground. The termgrounding strip as used herein means any connection that provides areturn path to ground. For example, the grounding strip may be a groundplane, a conductive strap, a conductive strip, a terminal strip, etc.

Once the flame 140 begins to oscillate, the ionization field surroundingthe flame will also oscillate. The electronic module 102 senses theoscillation and takes appropriate action if the oscillation magnitudeand frequency are above threshold levels as described herein. Turningnow to FIG. 7, the oscillations in pressure and in ion current areshown. In FIG. 7, curve 700 illustrates a pressure oscillation from apressure sensor mounted in a combustion chamber having an igniter 300used as an ion sensor. Curve 702 is the ion current flowing throughelectrode 302. It can be seen that the ion current can provide a directindication of pressure oscillations in the combustion chamber. FIG. 8,which is a fast Fourier transformation (FFT) of FIG. 7, illustrates thatthe dominant frequencies of the ion current 702 tracks the dominantfrequencies of pressure 700 over various operating conditions in thecombustion chamber 106.

When the flame 140 becomes unstable, it will typically exhibit pressureoscillations ranging in frequency from a few Hz to a few hundred Hz andhigher. Oscillations with amplitudes as low as ±1 psi are capable ofproducing audible noise that cannot be tolerated in some cases. Inaddition to noise, the pressure oscillation waves can create mechanicalstress in the system, leading to premature failure and even catastrophicfailure. The combustion chamber liner and turbine blades (not shown) aremost susceptible to high fatigue stress caused by combustionoscillations.

Turning now to FIG. 9, the steps the electronic module 102 performs indetecting the onset of combustion instability is illustrated. Setpoints(i.e., thresholds) are determined by an operator and are stored in anengine control module or other control module such as an ignitioncontrol module (step 900). The setpoints include oscillation magnitudeand frequency thresholds that the control module is to detect. Forexample, the thresholds could be for the onset of combustioninstability, a shut down level (e.g., destructive combustioninstability), etc. For purposes of explanation, two thresholds will beused. Those skilled in the art recognize that any number of thresholdsmay be used. The thresholds used for explanation are a first thresholdand a second threshold. The first threshold is for the onset ofcombustion instability where the oscillation frequency and magnitude arein a region where control parameters can be changed to move thecombustion system operation away from the unstable range. The secondthreshold is for conditions where emergency actions must be performedsuch as reducing the power or shutdown the system to protect the systembecause further operation can lead to serious mechanical failure.

The electrode 302 is energized at the appropriate point in the cycle(step 902). Typically, the electrode 302 is energized after (or when)the fuel/air mixture is ignited. Electronic module 102 receives the ionwaveform and processes the waveform (step 904). The waveform processingincludes detecting if there is any oscillation in the waveform. If thereis oscillation, the magnitude and frequency of oscillation isdetermined. If the oscillation magnitude is above the first thresholdand below the second threshold (step 906), the frequency is checked todetermine if it is within the frequency band setpoint for the firstthreshold (step 908). If the oscillation frequency is within thefrequency band, a notice is sent to the engine control module so thatcontrol parameters can be changed such that the turbine operates furtheraway from the point of combustion instability (step 910). It should benoted that the sequence of checking magnitude first and then frequencyis arbitrary and the frequency may be examined first and then themagnitude or both may be checked simultaneously.

If the oscillation exists, the module 102 determines if the oscillationmagnitude is above the second threshold level (step 912). If theoscillation magnitude is above the second threshold, the moduledetermines if the frequency is within the frequency band setpoint forthe second threshold (step 914). If the oscillation frequency is withinthe frequency band, an alarm is sent so that appropriate action can betaken such as shutting down the combustion system or derating the systemoutput to avoid damage to the combustion system (step 916). In somecontinuous combustion systems, the notice and/or alarm is sent if themagnitude is above the threshold or the frequency is within thefrequency band.

In response to receiving the signal, a mitigating action is taken suchas enriching the air/fuel ratio, adjusting the flow nozzle geometry, orother type of mitigating action. In one embodiment, an individual trimfunction is provided to each pilot nozzle of the engine to provide themitigating action of enriching the air/fuel ratio. Turning now to FIG.10, the pilot nozzle 950 is located at the centerline of the combustor106 surrounded by main fuel nozzles 952. The main nozzles 952 have airswirlers 954 that mix air with the injected fuel such that the resultingmixture is nearly homogenous. Over 90% of the total fuel is admittedthrough the main nozzles 952 in a premixed fashion, but the mixture istypically too lean to sustain stable combustion. The pilot nozzle 950admits up to 10% of the total fuel without being premixed with air. Thispilot fuel burns quickly in a diffusion flame at much higher temperaturethan the lean premixed flame and tends to stabilize and anchor theentire flame at a desired location in the combustion chamber 106. A trimvalve 956 is placed in the fuel line to the pilot nozzle 950 to adjustfuel flow and each trim valve is controlled by electronic module 102.The trim valve 956 is of limited authority, meaning that it only has theability to affect some small portion of the total flow in the pilot fuelcircuit. The main control of the pilot stage is handled by controller958 via a pilot stage fuel control valve located in the fuel skid 960.For purposes of explanation, the trim valve 956 will have a range ofadjustment of 10% of the pilot fuel flow from each pilot nozzle 950. Itis recognized that other ranges may be used. The trim valve 956 will beset at the midpoint of its adjustment range, 95%. This means that thetrim valve 956 has the ability to raise the pilot fuel flow to a givennozzle to 100% of available flow or lower the pilot fuel flow to a givennozzle to as low as 90% of available flow.

During operation, electronic module 102 receives the ion waveform fromelectrode 302 and processes the ion waveform to determine if combustionoscillations are of such magnitude or are in a frequency range such thatmitigating action must be taken to protect the engine or improvecombustor performance. If mitigating action must be taken, theelectronic module 102 determines if the proper mitigating action is toincrease the quantity of pilot fuel to the single combustor experiencinginstability or to every combustor. In most situations, the pilot fuel tothe single combustor experiencing instability will be adjusted. Themodule 102 obtains permission from the engine controller 958 to increasepilot fuel quantity. The main engine control 958 grants permission basedon predetermined rules. The module 102 drives the trim valve 956 to opena predetermined amount. In one embodiment, the predetermined amount is1% of flow. The additional pilot fuel flow raises the overall air/fuelratio and causes the flame to burn slightly hotter, thereby becomingmore stable, but at the expense of producing more NO_(x) emissions. Theelectronic module 102 waits for a period of time to determine if theflame stability is adequately stable by comparing the ion signal to thethresholds. If the flame has not yet achieved adequate stability asdetermined by the thresholds, the trim valve 956 is commanded to allowmore fuel to flow. For example, the trim valve is opened an additional1% to 97%. The additional pilot fuel flow raises the overall air/fuelratio and causes the flame to burn slightly hotter. This process isrepeated until the electronic module 102 determines that the flame isadequately stable (i.e., within the limits of normal combustion), and nofurther action is required. This process generally results in theminimum amount of pilot fuel being used. If the trim valve settingreaches an upper limit, the electronic module 102 sends a notice toengine controller 958 to take other action such as adding more fuel tothe system via the main nozzles 952, shutting the system off, etc. Theelectronic module 102 monitors the flame and makes a determination ofstability on a recurring basis. Once the flame is shown to be stable,the controller 958 goes through a periodic test to determine how muchexcess pilot fuel is being used. The controller commands electronicmodule 102 to command the trim valve to restrict the pilot fuel flow bya predetermined amount. The resulting air/fuel ratio will become leaner,and the flame temperature will be reduced. The electronic module 102measures the ion signal oscillations and compares the magnitude andfrequency against the thresholds. If the flame is still within thestable range, the process is repeated until the threshold of stabilityis achieved. The electronic module 102 continues to check for combustioninstability as described above.

Any malfunction of the stability control system only affects combustionperformance by the limited amount of control range of the trim valve956. Most control is still retained via the pilot control valve in thefuel skid 960. The safest failure mode for the trim valve 956 is fullopen, which will likely result in excessive pilot fuel. This willproduce excess NO_(x) emissions, but will likely not damage thecombustor by pressure oscillations or excessive high combustiontemperatures. While the electronic module 102 is manipulating the trimvalve of a single nozzle in a single combustor, the remaining pilotnozzles of the other combustors are largely unaffected. This allows allcombustors to be maintained in an optimum manner for flame stability,efficiency, and emissions. Those skilled in the art will recognize thata trim valve on the pilot nozzle is only one possible technique forclosed loop control of combustion instability. For example, anothertechnique is using a variable geometry nozzle, where the shape of thepremixer is changed to create a different fuel-air distribution.

It can therefore be seen that a method and apparatus to detectcombustion instability has been described. The need for a pressuresensor to sense combustion instability is eliminated using the presentinvention. Life-time maintenance costs of the turbine system is reducedwith the elimination of the pressure sensor. The control components maybe separately housed or be integrated into existing turbine controlmodules.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A system for detecting combustion instability in a continuouscombustion system having a combustion region comprising: at least oneretractable igniter having at least one electrode, the at least oneretractable igniter positioned at a location in the continuouscombustion system such that the at least one electrode is exposed togases in the combustion region; and a controller coupled to the at leastone electrode, the controller capable of receiving from the at least oneelectrode a combustion ionization signal and detecting an oscillation inthe combustion ionization signal indicative of the occurrence ofcombustion instability.
 2. The system of claim 1 wherein the controlleris adapted to monitor a temperature of the at least one retractableigniter and actively position the at least one retractable igniter totake ion measurements and retract the at least one retractable igniterif the temperature is above a threshold.
 3. The system of claim 2wherein the controller is further adapted to retract the at least oneretractable igniter if the temperature is rapidly approaching thethreshold.
 4. The system of claim 1 wherein the controller is adapted toposition the retractable igniter to keep the retractable igniter exposedto gases in the combustion region and below a temperature threshold. 5.The system of claim 1 wherein the controller is adapted to excite the atleast one electrode to create an electric field from the at least oneelectrode to a ground of the combustion region.
 6. A system fordetecting combustion instability in a continuous combustion systemhaving a combustion region comprising: at least one retractable igniterhaving at least one electrode, the at least one retractable igniterpositioned at a location such that the at least one electrode is exposedto gases in the combustion region, the at least one retractable igniterbeing retractable during operation of the continuous combustion system;and a controller coupled to the at least one electrode, the controllercapable of receiving from the at least one electrode a combustionionization signal and detecting an oscillation in the combustionionization signal indicative of the occurrence of combustioninstability.
 7. The system of claim 6 wherein the controller is adaptedto monitor a temperature of the at least one retractable igniter andactively position the at least one retractable igniter to take ionmeasurements and retract the at least one retractable igniter if thetemperature is above a threshold.
 8. The system of claim 7 wherein thecontroller is further adapted to retract the at least one retractableigniter if the temperature is rapidly approaching the threshold.
 9. Thesystem of claim 6 wherein the controller is adapted to position theretractable igniter to keep the retractable igniter exposed to gases inthe combustion region and below a temperature threshold.
 10. The systemof claim 6 wherein the controller is adapted to excite the at least oneelectrode to create an electric field from the at least one electrode toa ground of the combustion region.
 11. A method for detecting combustioninstability in a lean premix gas turbine having a combustion chamber andan electrode positioned in the combustion chamber at a location suchthat the electrode is exposed to combustion gases containing free ionsin the combustion chamber, the method comprising the steps of: applyinga voltage to the electrode during the combustion process; determiningparameters of a current flowing between the electrode and at least onewall of the combustion chamber of the lean premix gas turbine;determining if the parameters indicate the combustion process is one ofat the onset of combustion instability or is unstable; adjusting atleast one combustion system parameter if the parameters indicate thecombustion process is one of at the onset of combustion instability oris unstable.
 12. The method of claim 11 wherein the step determining ifthe parameters indicate the combustion process is one of at the onset ofcombustion instability or is unstable comprises determining that thecombustion process is at the onset of combustion instability if anoscillation frequency is within a predetermined frequency range and anoscillation magnitude corresponds to a first threshold.
 13. The methodof claim 12 wherein the first threshold corresponds to±1 psi.
 14. Themethod of claim 12 wherein the predetermined frequency range isapproximately±50 Hz of a critical frequency of the gas turbine.
 15. Themethod of claim 12 wherein the predetermined frequency range is betweenapproximately 10 Hz and approximately 10 kHz.
 16. The method of claim 12wherein the step of determining if the parameters indicate thecombustion process is one of at the onset of combustion instability oris unstable comprises determining that the combustion process isunstable if an oscillation frequency is within the audible frequencyrange and an oscillation magnitude corresponds to at least a secondthreshold.